Can I unfold not all things but only one thing in Coq?

Question

For example,

Example example (a b c : Q) : 
(a * b) * c == a * (b * c).
Proof.
unfold Qmult.

This code show me this screen.

1 goal
a, b, c : Q
______________________________________(1/1)
Qnum (Qnum a * Qnum b # Qden a * Qden b) * Qnum c
# Qden (Qnum a * Qnum b # Qden a * Qden b) * Qden c ==
Qnum a * Qnum (Qnum b * Qnum c # Qden b * Qden c)
# Qden a * Qden (Qnum b * Qnum c # Qden b * Qden c)

Can I unfold just one specific multiplication?

Answer 1

You can unfold the first multiplicaion by "unfold Qmult at 1" or the second by "unfold Qmult at 2".